The precise roles of H2-M in the immune system, in normal tissue development and maintenance, as well as in embryonal and fetal development, are not well known at this time. Due to the known involvement of H2-M in the presentation of exogenous antigenic peptides on major histocompatibility complex (MHC) class II molecules, H2-M proteins are important drug targets for modulation of immune responses.
The generation of H2-M modified transgenic animals would aid in defining the biological role(s) of H2-M, and produce an animal model of H2-M deficiency to be used in the design and assessment of chemical and biological approaches to modulating H2-M activity. Such H2-M modified transgenic animals can also be used as a source of cells for cell culture.
The function of MHC class II molecules is to present foreign antigens to T lymphocytes. MHC class II molecules at the cell surface are trimeric complexes of the alpha and beta chains associated with peptides derived from degraded proteins. During infection, the peptides are derived from the invading organisms leading to antigen-specific activation of T lymphocytes. T lymphocytes are also activated by non-self MHC class II molecules which is the basis of graft rejection.
MHC class II molecules are expressed mainly in antigen presenting cells which are derived from the bone marrow. There are three human MHC class II molecules, HLA-DP, HLA-DQ and HLA-DR, while the mouse has two MHC class II molecules, H2-A and H2-E. The process of antigen degradation and peptide association with MHC class II occurrs in the endosomal system of the antigen presenting cells. Intracellularly MHC class II molecules are associated with a third chain, the invariant chain. The invariant chain has several functions; it blocks binding of peptides and proteins to MHC class II in the endoplasmic reticulum (ER); it facilitates MHC class II transport out of the ER; and it directs MHC class II to endosomes where peptide loading occurs. Before peptide loading of MHC class II can occur the invariant chain has to be removed. Proteolysis and acidic pH leads to degradation and removal of most of the invariant chain, but a final fragment, called CLIP (class II-associated invariant chain peptides) cannot be removed by proteolysis, and exchange of this fragment for antigenic peptides is catalyzed by HLA-DM (in human, H2-M in mouse), which is an endosomal or lysosomal resident in MHC class II expressing cells. Cell lines lacking functional HLA-DM can only poorly process and present antigens and instead the MHC II molecules at the cell surface of these cell lines contain the CLIP peptide (Fling et al., 1994, HLA-DMA and -DMB genes are both required for MHC class Elpeptide complex formation in antigen-presenting cells. Nature 368, 554-8; Morris et al., 1994, An essential role for HLA-DM in antigen presentation by class II major histocompatibility molecules. Nature 368, 551-4). Reintroduction of HLA-DM into these cells restores the ability to process antigens showing that HLA-DM is important for modulating the peptide content of MHC class II molecules. Ln vitro experiments have shown that purified HLA-DM can directly mediate peptide exchange in purified MHC class II molecules, leading to exchange of CLIP or other poorly fitting peptides for well fitting peptides (Denzin and Cresswell, 1995, HLA-DM induces CLIP dissociation from MHC class II ad dimers and facilitates peptide loading. Cell 82, 155-165; Sloan et al., 1995, Mediation by HLA-DM of dissociation of peptides from HLA-DR. Nature 375, 802-806). The murine molecule, H2-M, can substitute for HLA-DM in human cell lines and in vitro, but no murine cell lines defective in H2-M have yet been published (Karlsson et al., 1994, Reconstitution of an operational MHC class II compartment in nonantigen-presenting cells. Science 266, 1569-1573; Morris et al., 1994, supra).
MHC class I and class II molecules are essential for the function of the immune system, since activation of T cells requires either one of the two classes of MHC molecules. In a normal situation, the immune system provides good protection against infectious agents and probably against tumor development. However, many pathological states also result from undesirable immune responses. Autoimmune diseases such as rheumatoid arthitis and systemic lupus erythematosus are typical examples of the self attacks by the deregulated immune system that lead to chronic inflammation and eventually the loss of function of the target organs. Rejection of grafts is another example of undesirable reactivity of the immune system in transplantation.
The present treatment options for chronic inflammatory disease are directed towards minimizing the effects of the inflammatory reaction. For severe cases such as organ transplantations, patients are treated with immunosuppressive drugs. These drugs are non-specific, however, since most reactivity of the immune system is decreased and the treated patients become susceptible to all kinds of infections. Many autoimmune diseases are associated with particular MHC class II alleles, although it is unclear exactly how this association is correlated with MHC class II-mediated antigen presentation. It is likely that the ability to modulate only MHC class II-mediated T cell activation could control most of the unwanted immune reactivity, while still leaving protection against infection through MHC class I-restricted T lymphocytes.